v0.5.2: Fix builds from source in .crate file

The crates uploaded for 0.5.0 and 0.5.1 were completely broken
for automatic builds because two things were excluded that shouldn't
have been:  An (empty) potentials/ directory, and the manpage file.

The manpage file is an easy fix.  Leaving behind an empty directory
for potentials/, however, appears to be impossible.  We have to create
a copy of the lammps source tree in our output directory just so we can
add an empty directory at build script runtime.

add readme to Cargo.toml

Fix release date

I'm not sure what interrupted me moments before deployment
a couple of months ago, but... yeah.

Fixes the fake mpi.h include so that it actually builds on systems wi…

…thout any systemwide MPI installation.

Update README.md